The objective of much fluidisation research has been the prediction of industrial beds, which are often large and operated at high pressure and temperature. This may be achieved through the use of Eulerian two-fluid models. Two such models have been compared with experimental results of the structure of a jet in a bubbling fluidised bed. In the first two-fluid model the particles are treated as a Newtonian fluid with a constant viscosity; in the second, kinetic theory for granular flow is used to describe the rheological properties of the particulate phase. The experiments were examined non-intrusively using X-ray equipment. The comparison of the experiments with the models revealed significant and systematic differences. These are described and the implications they have for the modelling and scaleup of bubbling fluidised beds are discussed.